1. Field of the Invention
The present invention relates to a liquid crystal display device which uses a cold cathode fluorescent lamp as a light source, and more particularly to the light emission efficiency improving structure of a phosphor film formed on an inner peripheral surface of the cold cathode fluorescent lamp and a manufacturing method of the phosphor film.
2. Description of the Related Art
In an image display device which uses a non-light-emitting-type liquid crystal display panel, an electronic latent image formed on the liquid crystal display panel is visualized with the provision of an external illumination means. With respect to the external illumination means, an illumination device is arranged on a back surface or a front surface of the liquid crystal display panel except for the structure which uses a natural light. The display device which requires particularly high brightness adopts, as a mainstream, the structure which mounts an illumination device on the back surface of the liquid crystal display panel. This structure is referred to as “backlight”.
The backlight is roughly classified into a side-edge-type backlight and a direct-type backlight. The side-edge-type backlight adopts the structure in which a linear light source represented by a cold cathode fluorescent lamp is arranged along a side edge of a light guide plate formed of a transparent plate. The side-edge-type backlight has been popularly used in a display device for a personal computer which is requested to satisfy a demand for the reduction of thickness thereof or the like. On the other hand, in a large-sized liquid crystal display device such as a display monitor or a display device used for a television receiver set, the direct-type backlight is popularly used. The direct-type backlight adopts the structure in which an illumination device is arranged directly below a back surface of the liquid crystal display panel.
In the general cold cathode fluorescent lamp used in this type of illumination device, a pair of cathodes is respectively arranged at both end portions of a light-transmitting glass tube, a phosphor film is formed on an inner peripheral surface of the glass tube by coating, and mercury and a rare gas are sealed in the inside of the glass tube. By generating an electric discharge in the inside of the glass tube, ultraviolet rays mainly having a wavelength of approximately 254 nm are generated by excitation radiation of mercury thus exciting phosphors with the ultraviolet rays whereby visible light is radiated leading to the acquisition of a luminous flux formed of emitted light.